Neuronal injury and microglial activation are related processes that appear central to a number of chronic neurodegenerative diseases including AIDS dementia. Ramified tissue microglia are highly branched, quiescent cells that secrete neurotrophic factors. Generally, neuronal damage can be an important early trigger in activating ramified tissue microglia to become phagocytic macrophage-like cells. Uncontrolled activation of microglia results in release of a variety of substances that are toxic to bystander neurons. Conversely, healthy neurons and certain soluble factors oppose microglia activation by signaling microglia to maintain a quiescent, ramified phenotype, downregulate production of neurotoxic substances, and upregulate production of neurotrophic factors. Fine regulation of this reversible microglial activation/ramification cycle limits neuronal injury during infection or insult and favors regenerative healing following injury to the CNS. Clearance of cellular reservoirs that harbor HIV during HAART treatment is mainly dependent on the particular turnover rate of the reservoir target cell population. Most of the HIV-1-infected cells in the brain are macrophages and microglia. Ramified tissue microglia in the CNS represent one of the most stable cell types in the body and have one of the slowest turnover rates of any HIV target cell population that has been measured. Virus could persist in these quiescent cells for years or perhaps even decades before the cells turn over. However, since poorly characterized cellular interactions and unknown soluble factors mediate and maintain microglial ramification in the brain and because they are inaccessible, ramified microglia remain poorly understood. The investigators have developed neuron-rich organotypic brain slice cultures and dissociated cortical brain cultures that support microglial ramification and visna lentivirus infection in vitro. This type of system offers a unique opportunity to investigate the biology of the lentivirus-infected ramified microglia in neuron-rich, simulated brain microenvironments conducive to experimental manipulation and mechanistic studies. Such model systems could provide important insights into HIV persistence in the CNS. The central hypothesis is that HIV-infected cells in the brain include ramified tissue microglia with latent or restricted infection. Replication competent provirus persists in this cellular reservoir for prolonged periods in spite of treatment due to the extremely slow turnover rate of these cells and its location behind the blood brain barrier. We will test these hypotheses by: 1) improving and characterizing primary culture models of microglial ramification, 2) determining the effect of lentivirus infection microglial ramification, 3) identifying changes in susceptibility to lentivirus infection that occur during microglial ramification, and 4) characterizing markers and modulators of ramification of infected and uninfected microglia.